This invention relates to an improved codec for accommodating the transmission of digital information between an analog modem and a digital modem through a digital network and, more particularly, to a codec which facilitates higher speed data transmission.
The public switched digital network (PSTN) was designed for voice transmission by sampling speech signals at an 8 kHz rate and encoding the sampled amplitudes into an 8-bit PCM code to provide a nominal channel capacity of 64 kb/s. However, the maximum achievable speed for data transmission is lower, for several reasons. Higher speed transmission can be effected through the use of techniques which raise the channel""s SNR, such as are disclosed in U.S. Pat. No. 5,394,437. That patent describes how some of the effects of quantization noise can be overcome by synchronizing the sampling times of the client modem, as well as its slicing levels, to those employed at the network""s PCM codec. Speeds approaching about 56 kb/s are then achievable between a digital modem that is directly connected to the digital switch by a digital line, such as T1 and an analog modem.
Although an 8-bit PCM code is transmitted through the PSTN at an 8 kHz rate, a small fraction of these bits are often used to provide signaling information (e.g., switchhook state, dialing and call progress status). Such signaling is called xe2x80x9crobbed bitxe2x80x9d or channel-associated signaling because the signaling bit displaces the least significant bit of the voice signal once every six samples. While the degradation of voice quality is tolerable, this technique additionally causes a loss of approximately 1.33 kbps from a modem""s data rate.
At the network interface, analog voice signals, which typically exhibit a large amplitude range, are converted to the 8-bit digital PCM code by an A/D converter called a codec. However, in order to properly accommodate the large amplitude variation of voice signals, a quasi-logarithmic rule of conversion (the xcexc-law or A-law rule) is employed. According to xcexc-law conversion, unit step sizes are employed in the conversion of the sixteen smallest amplitude signals but step sizes, as great as 128 times as large, are employed to encode largest amplitude voice signals. A complete list of the 128 slicing levels employed in xcexc-law conversion is set forth in FIG. 1 of U.S. Pat. No. 5,406,583. Although this approach is appropriate for keeping the signal to noise ratio (SNR) relatively constant regardless of amplitude for voice signals, it limits the achievable data rates for xe2x80x9c56 kxe2x80x9d or xe2x80x9cv.pcmxe2x80x9d modems to under 56 kbps. This is because it is extremely difficult for the analog xe2x80x9cv.pcmxe2x80x9d modem to distinguish low amplitude signals that are separated by one unit in the presence of residual intersymbol interference from neighboring symbols that may be 4000 times larger in amplitude.
It would be extremely advantageous to further overcome the effects of the quantization noise introduced by the network""s PCM codec and thereby achieve higher transmission speed than the-less-than-56-kbps rate that is currently achievable by xe2x80x9cv.90xe2x80x9d or xe2x80x9c56 kxe2x80x9d modems.
In accordance with the principles of the present invention, in one illustrative embodiment thereof, the quantization levels of the network codec are made more suitable for data transmission rather than for voice communication. The digital modem will send a signal to the network codec that will cause the codec to recognize that a data call is being made that involves a connection to a digital modem. Upon recognizing that a data call is involved, the codec changes its mode of operation so that it discontinues using the xcexc-law (or A-law) conversion rule that it employs on conventional voice calls and, instead, linearly converts the amplitudes of the analog data signal into equivalent digital codes as more commonly used in data communications. Advantageously, this recognition may take place during the transmission of the 2100 Hz answer-tone-with-phase-reversal (as specified in ITU-T Recommendation G. 165), that is sent by a modem incident to its being connected for use on a call. In the prior art this 2100 Hz tone was sent by a modem as a signal for the network to disconnect or bypass its echo cancellers in addition to providing an answer signal to the calling modem. In accordance with the present invention, the 2100 Hz tone will continue to be used for this purpose. However, the 8-bit digital PCM code that represents the instantaneous amplitudes of the 2100 Hz tone will not all be devoted to the generation of the tone. Instead, when a digital modem is connected, one or more of the least significant bit positions of the code representing the 2100 Hz tone can be allocated (xe2x80x9crobbedxe2x80x9d) without materially detracting from the accuracy of the 2100 tone to signal to the associated codec that a digital modem connection has been made. These bits are then used to provide an indication to the network codec that a PCM modem connection is being made so that the codec can switch from xcexc-law or A-law coding to linear coding.